According to Microsoft corp, computers capable of running Windows Vista
are classified as Vista Capable and Vista Premium Ready.

A Vista Capable or equivalent PC needs to have at minimum an 800 MHz
processor, 512 MB RAM and a DirectX 9 class graphics card. A computer that
meets these requirements will be capable of running all editions of
Windows Vista although some of the special features and high end graphics
options may require additional or more advanced hardware.

A Vista Premium Ready PC will take advantage of Vista's "high-end"
features but will need at least a 1.0 GHz processor, 1 GB main memory, and
an Aero-compatible graphics card with at least 128 MB graphics memory and
supporting the new Windows Display Driver Model. The company also offers
Windows Vista Upgrade Advisor from its website to determine the ability of
a PC to run Vista in its various guises. The utility runs on Windows XP
(with Service Pack 2) and Windows Vista.
Microsoft lists some Vista capable hardware on their website. The "Windows
Vista Premium Ready" laptops they specify have Intel Core 2 Duo T5500 or
above CPUs and 1 GB memory.

Windows Vista's "Basic" and "Classic" interfaces will work with virtually
any graphics hardware that supports Windows XP or 2000; accordingly, most
discussion around Vista's graphics requirements centers on those for the
Windows Aero interface. As of Windows Vista Beta 2, the NVIDIA GeForce FX
family and later, the ATI Radeon 9500 and later, Intel's GMA 950
integrated graphics, and a handful of VIA chipsets and S3 Graphics
discrete chips are supported.Though some XGI Technology Volari chips were
DirectX 9 (including the Volari V3XT which was available in PCI cards),
with XGI's exit from the graphics card business it appears none of its
chips are supported as of Vista Beta 2. A PCI Express (PCIe) video card is
not a requirement for Windows Aero, but Microsoft recommends PCIe video
over an AGP device due to the interface's greater bandwidth. There are
some PCI cards available that are compatible with Windows Vista as well.